1. Field of the Invention
The present invention relates to hydraulic equipment having a plurality of pumps each connected by a control valve arrangement to a plurality of hydraulic functions; and in particular to a method for allocating the flow of fluid from each pump to the plurality of hydraulic functions.
2. Description of the Related Art
Hydraulic systems for large machines, such as an earth excavator, often incorporate a number of hydraulic pumps in order to satisfy the demand for pressurized hydraulic fluid to drive various hydraulic actuators. A hydraulic actuator is a device, such as a cylinder-piston arrangement or a hydraulic motor that converts the flow of hydraulic fluid into mechanical motion. Because several of these hydraulic actuators on the machine can be operating simultaneously, the aggregate demand for hydraulic fluid flow is greater than can be provided by any single, reasonably sized, pump. In some previous hydraulic systems certain pumps were assigned to only selected ones of the hydraulic actuators and thus could not supply hydraulic fluid to all of the hydraulic actuators on the machine. This fundamental arrangement often produced an inefficiency when the demand for the hydraulic fluid from one group of actuators could not be satisfied by its permanently assigned pump and fluid was available from the other pump but could not be supplied to the demanding hydraulic actuators.
Other hydraulic systems allowed multiple pumps to supply fluid to the same hydraulic actuator. Nevertheless, the previous technique for doing so provided a fixed algorithm for each hydraulic actuator that defined for any given amount of fluid demand how that demand was satisfied with fluid from the two pumps. For example, at relatively low flow demands all of the demand was satisfied by fluid from one of the pumps until the demand reached a given percentage (e.g., 50%) of the maximum flow that could be produced by the pump. Thereafter, the additional flow requirements for the hydraulic actuator was satisfied by a combination of fluid from both of the pumps according to a predefined proportional relationship. That apportionment of fluid from the different pumps to any given hydraulic actuator was fixed and was not a function of the demands for fluid from other hydraulic actuators on the machine. In other words, regardless of whether any of the other hydraulic actuators also were operating simultaneously, the apportionment of fluid to each actuator was fixed and did not take into account the demand from other actuators. Therefore, an operator of the machine could command the activation of one hydraulic actuator which would begin to operate and then the operator could command another hydraulic actuator to begin operating simultaneously and the proportioning of fluid from the various pumps to each actuator was affected only by the operator command for that actuator and did not take into account the simultaneous commands for the other actuators. Such an independent operation of each actuator did not always produce the most efficient and energy conserving operation of the overall machine.
In order to produce optimal efficiency, the various pressure requirements of the different simultaneously operating actuators has to be taken into account. It has been commonplace on machines with multiple actuators to sense the load force on all of the hydraulic actuators and select the greatest of those load forces to use in controlling the outlet pressure of the pump. It's a fundamental concept that in order for an actuator to be able to move its load, the pressure of the hydraulic fluid applied to that actuator must produce a force that exceeds the force produced by that load. This often results in a hydraulic actuator that has a relatively small load acting thereon receiving pressure far greater than that which is required to move that load. As a consequence, when the fluid at that high pressure flows through a small opening in the control valve associated with this small hydraulic load, high heat losses are produced which therefore contribute to the inefficient operation of the hydraulic system. Therefore, to improve the efficiency of the overall machine, it is desirable to match the pump outlet pressure as closely as possible to the pressure requirements of the various hydraulic functions. Doing so minimizes the heat losses at each valve assembly. Many previous systems did not factor in the pressure requirements when allocating fluid to the different hydraulic functions.